The invention pertains to the field of pump systems for liquid chromatography systems or other systems where it is necessary to obtain a steady flow of liquid at high pressure with an accurate composition of solvents carrying the sample to the column. More specifically, the invention pertains to the field of control systems for high pressure pumps which take into account the compressibility of fluids at high pressure in controlling solvent inlet valves to get the desired proportions of solvents used as the carrier balanced correctly regardless of variations in solvent compressibility.
Liquid chromatography systems are characterized by columns filled with small beads coated with active chemicals. Through these columns solvents are pumped which carry, in solution, samples of unknown composition which are to be assayed. These solvents may hereafter be referred to as carriers, and typically are delivered at pressures from 1,000 to 6,000 psi. It is characteristic of liquid chromatography systems that the different components in the samples move through the column at different rates of speed. Thus when a solvent carrying a sample of unknown composition of multiple chemicals is forced through the column, the different rates of movement will result in the various components emerging from the other end of the column separated in time. The identities and quantities of the various components of the original sample composition may then be determined.
Good control over the flow rate of the solvent carrying the sample through the column extends the life of the column and leads to better assay results. That is, constant flow over a long term is necessary for repeatability of the retention times of the chromatogram peaks and is a factor in the area of the peak.
The solvent must be pumped through the column at very high pressures to get a practical flow rate through the column because of the high degree of restriction to flow imposed by the column. Many solvents such as certain alcohols are appreciably compressible at these high pressures. Typically, the flow rate through the column is assumed from the number of strokes taken by the pump and the known displacement on every stroke. Thus, if the solvent compresses at high pressure the assumed volume of flow through the column calculated from the number of pump strokes taken is not the true flow volume. To further complicate the problem, it is often unknown how much any particular solvent or any mixture of solvents will compress at a given pressure, the pressure may be anywhere from 50 p.s.i. to 6,000 p.s.i., and the solvent may be composed of a mixture of only one solvent, two or more solvents in a predetermined mix or a predetermined mix of solvents at the start with a gradual change to another predetermined mix at the finish of the assay. The compressibility may be as much as 5% so errors this large can be created.
Further, to mix two different solvents using the pump which pumps the solvents through the column, it is important to know how much of each solvent is being drawn in so as to properly mix the solvents in the user defined mix proportions. A problem arises because the pressure inside the pump when the pump piston is at top dead center and ready to begin drawing in solvent is still at the column pressure which is very high. There is always liquid around the piston when it is at top dead center which forms a dead volume, and this dead volume of liquid is still compressed. Thus when the piston starts down on its intake stroke to draw in solvent, the dead volume liquid expands into the newly created volume and no new solvent is drawing into the cylinder. This creates a "front end" error of unknown proportion in that the assumption is not true that the amount of solvent drawn in on the intake stroke is equal to the displacement of the piston during the stroke. Typically, the pistons used in the pumps for liquid chromatography systems are 1/8 inch in diameter and moves only 1/4 of an inch. Some companies use pistons which are 1/4 inch in diameter or 3/16 inches in diameter. Thus, errors created by compressibility of the dead volume can loom large as a percentage of the total piston displacement.
Liquid chromatography columns are not well suited to handle pressure pulses in the fluid flow therethrough. Such pressure pulses result in shortening of the life of the columns. In response to this need, pumps have been developed for liquid chromatography systems which use a two piston arrangement to dampen pulses and to keep a continuous flow of fluid through the column. One such pump is described in U.S. Pat. No. 4,552,513. In such pumps two pistons are used, one of which is called the pumping piston and the other of which is called the damping piston. The two pistons are driven by a common drive train and each has its own cam which causes the stroking motion for that piston. The cam of the pumping piston is such that the stroke movement if used alone would deliver up to four times the desired flow rate during the time the piston moved up in the cylinder. The damping piston cam is shaped so that this piston moves oppositely to the movement of the pumping piston movement. When the pumping piston is moving up in the cylinder, the damping piston is moving down at three times the desired flow rate. The shape of the damping piston driving cam is such that if the damping piston were used alone to deliver solvent, the flow resulting therefrom would be equal to the desired flow rate. There is check valve at the input to the pumping piston and between the output of the pumping piston and the input of the damping piston. The result of the combined movement of the two pistons is that as the pumping piston displaces fluid at the rate four times the desired flow rate, the damping piston takes in fluid at three times the desired flow rate. The resulting output flow rate is equal to the desired rate. As the pumping piston reaches the top of its stroke and begins to move downward for the input stroke, the damping piston begins to move upward to push fluid out the output at a rate equal to the desired flow rate. The result is that a constant flow at the desired rate appears at the output of the pump if the solvents being pumped are not compressible. However, if the solvents are compressible, a lag in the flow occurs during the time the pumping piston is compressing the solvent taken in at low pressure to the pressure level of the high pressure line coupled to the column. No flow appears during this lag because until the pressure of the fluid in the pumping piston cylinder reaches the high pressure level, the check valve between the pumping piston and the damping piston does not open. Since this is the path to the output, no flow occurs at the output and an undesirable pressure pulse occurs.
Accordingly, a need has arisen for a pump control system for liquid chromatography systems and other systems operating at high pressure where a constant flow rate is needed which can maintain a constant flow rate at all pressures and for all compressibility factors.